Generally, sonar devices are classified roughly into active sonar devices and passive sonar devices.
The active sonar devices are devices which emit a sound wave into the water and detects a reflected sound from a target (for example, a ship, a submarine, an underwater mine, and the like), thereby measuring the distance to the target and the location (direction) of the target from, respectively, the time difference between the emitted and reflected sounds and the direction in which the reflected sound is detected.
The passive sonar devices are devices which capture a sound wave emitted by a target to be the monitoring subject, which is present in or on the water, thereby detecting the existence and location (direction) of the target.
Sounds emitted by such a target include a stationary sound lasting for a relatively long period of time, whose emission source is an engine or the like, and a transient sound which is not continuous in time or lasts for only a short period of time, such as a steering sound and a sound of opening or closing a hatch.
Further, sounds in the water include not only the above-mentioned ones emitted by a target but also noises emitted by innumerable sound sources other than the target. Such noises can be categorized into stationary noises and transient noises, where the stationary noises include, for example, a sound caused by the rain, and the transient noises include, for example, a sound originating from an aquatic organism, atmospheric noises and the like. The atmospheric noises are electromagnetic wave noises caused by lightning or the like.
The passive sonar devices convert a received sound, including a sound emitted by a target and noises, into an electrical signal (analog signal) and, after performing predetermined preprocessing on the electrical signal, convert it from an analog signal into a digital signal. The preprocessing includes, for example, signal amplification, filtering and the like. The received signal (RAW signal) having been converted into a digital signal is further subjected to predetermined signal processing, and is subsequently displayed at a display section and also reproduced as a sound by a listening section.
From the image displayed at the display section and the sound reproduced by the listening section, an operator of the passive sonar device determines the presence or absence of a target signal corresponding to a sound emitted by the target. In that case, the operator needs to detect a target signal from the received signal including a large number of noises. Therefore, to assist the target signal detection by the operator, the passive sonar devices perform signal processing for analyzing the received signal and thereby discriminating between the target signal and the noises, and for removing (suppressing) the noises.
As analysis methods of such a received signal, there are known a method of performing frequency analysis of the received signal by the use of Fast Fourier Transform (FFT) or the like, and a method of performing power analysis by detecting the level of the received signal.
For the frequency analysis, for example, used is a well-known Short-time FFT which calculates variation with time of a signal's frequency spectrum by cutting out a signal segment with a constant interval in the direction of the temporal axis, then performing Fourier Transform on the signal segment, and repeating the transformation with the time period of a signal segment to be cut out being shifted. The power analysis is used for extraction of a transient signal corresponding to a transient sound emitted by the target or to a transient noise.
Generally, it is known that transient noises originating from an aquatic organism are generated in a low frequency range. Therefore, the transient noises originating from an aquatic organism can be removed relatively easily by the use of a bandpass filter or the like. However, noises such as atmospheric noises and an intentionally emitted pulse-shaped oscillation sound (hereafter, referred to as an active pulse oscillation sound) cannot be removed by the use of a filter, because their frequencies cannot be identified or their frequency range overlaps with that of the target signal. An example of such an active pulse oscillation sound is an oscillation sound for target detection which is emitted from an active sonar device arranged in the vicinity of the passive sonar device.
A technology of extracting transient signals from a received signal by using the power analysis and then detecting, from the extracted signals, a transient signal emitted from a target (hereafter, may be referred to as a target transient signal) is described in, for example, Patent Literature 1 (PTL 1). PTL 1 describes a method of discriminating the extracted transient signals into a target signal and noises by the use of a sound discrimination method combining well-known HMM (Hidden Markov Model) and NN (Neural Networks) methods.
Patent Literature 2 (PTL 2) points out a drawback of the power analysis in that a target transient signal cannot be recognized by the power analysis if a large power noise exists outside the band of the target transient signal, and accordingly proposes a technology for overcoming the drawback. PTL 2 describes a technology which performs an FFT process on a received signal, thereby normalizing the signal in terms of each frequency, and also calculates the direction of a sound source from the received signal, and then adds the normalized signal and the direction signal. By performing such a process, it becomes possible to display the amount of change in the level of a transient signal without depending on the frequency characteristics.
While it is an invention relating to a radio wave detection device instead of a sonar device, Patent Literature 3 (PTL 3) describes, as a method for suppressing impulsive noises (atmospheric noises), a method of processing a received signal after dividing it into a plurality of signal segments in terms of predetermined bands.
Patent Literature 4 (PTL 4) proposes a technology for detecting a target signal from received signals, which focuses on that a sound emitted by a sound source (target) present underwater has a high directivity in its arrival direction. PTL 4 describes a technology which calculates the direction vector of a transient signal extracted from a received signal and, if the magnitude of the direction vector (directional concentration degree) is equal to or larger than a preset threshold value, determines the transient signal to be a target signal.
Further, Patent Literature 5 (PTL 5) describes a technology for suppressing noises superimposed on an audio signal in, for example, cellular phones and the like. PTL 5 describes a technology which saves information on noises to be suppressed (noise information) in advance, generates mixed noise information (pseudo noise information) on the basis of a result of analysis of an audio signal including noises, and suppresses the noises using the mixed noise information.
Patent Literature 6 (PTL 6) describes an integration circuit which removes noises from time series data by integrating components of the same frequency, a direction detection circuit as a direction detecting means for calculating a signal direction, a directional variance calculation circuit as a means for calculating from the calculation result a variance d of the signal direction, and a directional variance weighting circuit as a means for applying an enhancive weighting to a signal direction having a small value of the variance d. PTL 6 also describes that sound signals are acoustically outputted from a speaker, and a gray-scale image drawing circuit displays the enhancively weighted signal directions in a gray-scale image on a sonagram.
As a method for improving S/N (Signal to Noise) of audio signals, for example, a technology described in Non-patent Document 1 (NPL 1) will be mentioned. In NPL 1, a method of suppressing background noises (stationary noises) by estimating a noise floor level from the level of an audio signal is shown.